Microchannel networks can include a multitude of interconnected passageways. These microchannel networks are often used in microfluidic systems. A more complete description of microchannel networks and their application in microfluidic devices may be found in Anderson, J. A., et al., Fabrication of topologically complex three-dimensional microfluidic systems in PDMS by rapid prototyping. Anal. Chem. 74, 3158-64 (2000).
Conventional microchannel devices are constructed by multiple methods, including laser machining, laser chemical processing, sacrificial wax, soft lithography, photopatterning, fused deposition, and two-photon polymerization. Two-dimensional microchannel devices are generally made by photolithographic or soft lithographic techniques and are limited to patterns on a flat surface, or at most a few stacked layers. Forming these devices requires repetitive lithographic processing, in which each layer requires a separate mask or stamp. Multiple series of plates may be joined to form structures having a few vertical layers.
These devices are made by etching open troughs into separate plates. Due to the limitations of lithography, the sidewalls of the etched troughs are straight. These plates are then joined, such as with an adhesive, so the open troughs align to form closed microchannels having square or rectangular internal shapes.
The approximately 90° corners of the square or rectangular microchannels provide many locations for stress cracks to form due to stress concentration at the corners. Since structures incorporating lithographically formed microchannels have a tendency to crack, square or rectangular microchannels are unsuitable for use in structural composite materials. Furthermore, structural materials, such as epoxy based materials, cannot generally be etched using lithographic methods.
In addition to these square or rectangular microchannels weakening materials in which they are incorporated, the corners provide areas for solids to collect. In this fashion, when colloids or other solid containing fluids are passed through the device, some of the solids collect in the corners. This build up of solids can result in decreased fluid movement through the device, in addition to plugging.
There is a need for self-healing structural materials. Structural thermosetting polymers and fiber reinforced polymer composites, which are used in a wide variety of applications ranging from microelectronics to composite aircraft structures, such as fuselages, wings, and rotors, are susceptible to damage in the form of cracking. These cracks can form deep within the structure where detection is difficult and repair is virtually impossible.
Conventional self-healing or self-sealing materials use a microencapsulated healing agent and a dispersed catalyst inside a polymer matrix to repair themselves. These self-healing materials are able to recover approximately 75% of the toughness of the original material prior to cracking. However, the use of these materials is limited because they can deliver the healing agent into the crack plane only once.
In addition to improved self-healing materials, there is a need to exert greater control over fluid flow and mixing in microchannel devices. Control over fluid flow and mixing is difficult in microfluidic devices because laminar flow and diffusive mixing are the dominant mixing modes. These problems are of particular concern for mixing fluids that contain biological or other large molecules, such as DNA or proteins, because such species diffuse slowly. In these devices, prohibitively long path lengths are often required to ensure complete mixing of the fluid constituents.
To reduce the planar footprint of such devices, recent efforts have focused on various design strategies for fluid mixing based on chaotic advection. Chaotic advection is believed to promote rapid stretching and folding of the fluid interfaces that are believed to exist within complex fluid flow patterns. A more detailed description of chaotic advection can be found in Aref, H., “The development of chaotic advection.” Phys. Fluids 14, 1315-25 (2002).
It is believed that chaotic advection is created in a fluid flow by either causing unsteadiness in the rate of fluid flow, or by providing geometrically complex channels to direct the fluid. By exploiting this phenomenon on the micro-scale, the interfacial surface area across which diffusion occurs is thought to greatly increase. Prior strategies of fabricating microfluidic devices believed capable of chaotic advection include fluid direction channels having “twisted pipe architectures” and devices having bas-relief structures imprinted along the floor of the fluid direction channels. While these methods may result in enhanced mixing, the complexity of the devices is limited due to the planar nature of the devices and the rectangular features obtained.
As can be seen from the above description, there is an ongoing need for simple and efficient materials and methods for forming microchannel-type devices, including microfluidic devices used for mixing and materials with the ability to self-heal. The microcapillary devices, fabrication methods, and materials of the present invention overcome one or more of the disadvantages associated with conventional devices.